The present invention relates to an electrolytic copper plating method used for electrolytically plating copper on the surface of a catalyzed, electrically nonconductive substrate for printed circuit boards, etc.
So far, an electroless copper plating bath containing formaldehyde have been industrially available for depositing a copper film on an electrolytically nonconductive plastic substrate, e.g., epoxy resin. Then, copper electroplating is carried out. This electroless copper plating bath is widely used for printed circuit board production, e.g., for forming an electrically conductive film of copper in the throughhole in a printed circuit board as the material to be plated with copper.
However, this electroless copper plating bath has the following problems.
(1) It includes formaldehyde said to be carcinogenic. PA1 (2) Its analysis and control are troublesome. PA1 (3) It is required to bail out a plating solution from the bath as much as a fresh one is replenished thereto. PA1 (4) Some limit is placed on to what degree the bath is loaded; that is, it is impossible to fill the bath up to the brim with the substrate to be treated. PA1 (5) It poses a defective deposition problem due to hydrogen gas being trapped in minute holes. PA1 (6) Much time is needed for plating. PA1 (7) Cost reductions are difficult to achieve. PA1 (1) Pd--Sn addition type, PA1 (2) Carbon black addition type, and PA1 (3) Organic conductive film addition type. PA1 Substrate (copper-clad laminated board) PA1 Drilling PA1 Catalyzing PA1 Drying PA1 Polishing PA1 Alkali-soluble dry film imaging (or resist ink printing) PA1 Degreasing PA1 Water rinsing PA1 Soft etching PA1 (Accelerator) PA1 (Water rinsing) PA1 Electrolytic copper plating PA1 Water rinsing PA1 Acid dipping PA1 Water rinsing PA1 Copper sulfate electroplating PA1 Water rinsing PA1 Acid dipping PA1 Solder electroplating PA1 Water rinsing PA1 Alkali-soluble dry film stripping PA1 Etching PA1 Solder stripping PA1 Solder mask coating PA1 HAL (hot air leveller)
In order to solve these problems, several methods without recourse to electroless copper plating have been proposed in the art. These methods (usually called the direct plating methods ) are generally broken down into the following three types.
However, printed circuit boards produced by these methods have serious problems in connection with the adhesion between the copper foils and the plated deposits, the reliability of heat-resistant testing, etc.; in other words, there are only a few examples in which they have been successfully used on an industrial scale.
A method of applying electrolysis to an electroless copper plating, high-alkaline bath containing formalin has already been reported by Honma et al (see the preprints at the 4th Lecture Meeting of the Printed Circuit Board Society). The purpose of applying electrolysis to this bath is to increase the deposition rate of electroless copper plating much more and to make improvements in the properties of the deposited films. Again, this method is unpreferred in view of environmental hygiene because of the use of harmful formalin.
A typical electroless copper plating bath making no use of formalin is a dimethylamineborane bath developed by LeaRonal Inc. (see U.S. Ser. No. 314,537, now U.S. Pat. No. 4,877,450, or Japanese Patent Laid-Open Publication No. 2-305971). This bath is improved in terms of its stability and the film properties of deposits by using two chelating agents, and is used at the bath temperature of 51.7.degree. to 73.9.degree. C. and in the pH range of 6 to 9.5. Another typical electroless copper plating bath making no use of formalin is a hypophosphorous acid bath developed by MacDermid Inc. For this bath electrolysis is imperatively needed, because the deposition of electroless copper plating is not initiated even at a bath temperature of 50.degree. to 60.degree. C., unless it is electrically energized just after dipping. In addition, minute amounts of cobalt and nickel are added to this bath so as to effect self-deposition reactions.
A conventional process of making printed circuit boards--which has some relation to this invention--is set forth in Laid-Open Japanese Patent Publication No. 60-213085. According to the disclosure, a conductive film of copper is formed by electroless copper plating after drilling, catalyzing, polishing and imaging using dry film are carried out on a substrate, then copper and solder electroplating are carried out. Because of using electroless copper plating, this method has the aforesaid several problems as well.
In view of the problems associated with conventionally used electroless copper plating and several direct plating methods proposed so far in the art, an object of this invention is to provide an electrolytic copper plating method using a reducing agent, which can be used in place of conventional electroless copper plating, dispense with any harmful substance such as formaldehyde, be easily analyzed and controlled, form an electrically conductive film of copper on an electrically nonconductive substrate with improved adhesion thereto, and is effectively applicable to making printed circuit boards in particular.
As a result of intensive studies made to achieve the object mentioned above, we have now discovered that an electrically conductive film of copper can be well formed on an electrically nonconductive substrate material by carrying out electrolysis under constant pH and temperature conditions with the application of an electrolytic copper plating bath using a reducing agent, and so have accomplished the present invention.